The present invention relates to novel phenolic resins and products derived therefrom which have improved thermal and oxidation stability. The resins do not give volatile products upon curing and do not involve the use of formaldehyde or formaldehyde-producing agents in their preparation.
Phenolic resins are well known and are generally prepared from phenols and formaldehyde. Such phenolic resins are cured by further reaction with formaldehyde or formaldehyde-producing compounds and/or by the residual methylol groups in the resin. The use of formaldehyde introduces environmental and toxicological problems in the preparation, fabrication, and even in the long term use of such materials. The reaction to form such resins introduces methylene bridges between the phenolic rings. These are points of attack for high temperature oxidative degradation. Most importantly the curing of conventional phenolic resins evolves water, alcohol and/or formaldehyde. This complicates the fabrication and these volatiles lead to voids in the cured resin. Such voids are highly detrimental to the mechanical properties of the resin, especially in high performance composite materials and adhesives such as those used in automotive, aircraft, and aerospace applications.
Recently, improved methods and processes for obtaining high molecular weight phenolic resins by enzymatic oxidation have been described in commonly assigned U.S. Pat. Nos. 4,647,952 and 4,900,671. These phenolic resins have the general formula ##STR4## where Y is present in the meta or para position and is selected from the group consisting an alkyl group, a halogen atom (e.g., fluorine, chlorine or bromine), an aryl group, a phenylalkyl group, an alkoxy group, an allyl group, a group of the formula --COOR where R is hydrogen or a phenylalkyl group, an amino group of the formula --NR.sub.1 R.sub.2 where R.sub.1 and R.sub.2 are the same or different and represent a hydrogen atom or an alkyl group; Z is a hydrogen atom, an alkyl group, a halogen atom, an aryl group, a phenylalkyl group, a --COOR group, or Z in conjunction with the adjacent meta position represents a condensed benzene ring which may be substituted or unsubstituted; and n is greater than or equal to 2.
Phenolic resins prepared by the oxidative coupling of phenols are generally superior to those phenolic resins prepared using formaldehyde. However, they do not have an inherent curing mechanism which is desirable in order for these resins to be useful for the preparation of composites, moldings, adhesives, coatings, and the like.
These resins have the phenolic units coupled directly to each other rather than through a methylene bridge as in conventional phenolic resins. Such systems overcome many of the disadvantages of conventional phenolic systems. However, to be useful for the preparation of composites, adhesives, moldings, or coatings they need to be "cured" or cross-linked, preferably without evolution of volatiles.